Output stages of electronic circuits, and especially of amplification circuits, conventionally have a low output impedance. This distinguishes an output stage from the stages of preceding ranks of the circuits, which may have high-impedance outputs. Such an output stage enables providing a load coupled at the circuit output with a fixed voltage and a current adapted to the power consumption of the load. Similar circuits may be used to form voltage division circuits.
FIG. 1 illustrates a circuit 10 behaving as a voltage divider. Circuit 10 comprises a first branch coupled between two terminals Vdd and GND across which is applied a D.C. power supply voltage, terminal GND being a ground terminal. The first branch comprises a series connection, from terminal Vdd, of a resistor Ra1, of an N-channel MOS transistor Na1, of a P-channel MOS transistor Pa1, and of a resistor Ra2. Transistors Na1 and Pa1 are diode-connected, that is, the gate and the drain of transistor Na1 are interconnected, and the gate and the drain of transistor Pa1 are interconnected. The first branch forms a voltage division circuit, also commonly referred to as a voltage divider.
A second branch comprising a series connection, between terminals Vdd and GND, of a transistor Na2 connected to form a current mirror with transistor Na1, that is, where the gates of transistors Na1 and Na2 are interconnected, and of a transistor Pa2 connected to form a current mirror with transistor Na2, is placed in parallel with the first branch. An output terminal OUTa of the circuit corresponds to the connection node between transistors Na2 and Pa2. A load, a capacitor Ca in the example of FIG. 1, is coupled between output terminal OUTa and ground GND.
Circuit 10 operates as follows. In the first branch, the voltage at a node Aa located between transistors Na1 and Pa1 is substantially equal to a ratio (Ra2×Vdd)/(Ra1+Ra2), to within the threshold voltage differences between transistors Na1 and Pa1. The current mirrors enable the current flowing in the first branch to be present in the second branch, proportionally to the ratio between terms W/L of the mirror transistors, W and L respectively being the width and the length of the considered transistor, and also enable the voltage on output OUTa to be substantially the same as that at node Aa.
When the load consumes power, a current proportional to this power consumption is provided thereto by the second circuit branch. This circuit operates properly as long as voltage Vdd is greater than the sum of the threshold voltages of transistors Na1 and Pa1.
A disadvantage of the circuit of FIG. 1 appears when this circuit is used with a so-called low power supply voltage Vdd. This situation is more and more frequent. Indeed, it is currently desirable to form circuits consuming less and less power. To achieve this, a solution comprises powering the circuits by means of so-called low power supply voltages. This is possible since electronic components also require less and less power to operate properly. However, the threshold voltages of MOS transistors is decreasing as technology advances, but not as fast as the decrease in circuit power supply voltages.
Thus, when the power supply voltage is decreased, the current in the first branch decreases but the voltage drop across transistors Na1 and Pa1 remains substantially constant. A problem is posed when the power supply voltage reaches a value only slightly greater than the sum of the threshold voltages of transistors Na1 and Pa1. In this case, the current flowing in the first branch, and thus in the second branch of the circuit, decreases and may not be sufficient to power load Ca.
Further, although N-channel MOS transistors having identical characteristics and P-channel MOS transistors having identical characteristics can now be formed on a same circuit, deviations still appear from one circuit to another between MOS transistors of the same type. If voltage Vdd is decreased, the current in the first circuit branch may vary according to the threshold voltages of transistors Na1 and Pa1, from one circuit to another.
To check whether a circuit such as that in FIG. 1 operates properly, simulations are performed by forcing the threshold voltages of the circuit transistors to extreme values. Such simulations enable testing the most unfavorable cases, especially when transistors Na1 and Pa1 have significant threshold voltages (slow transistors). In this case, the current in each of the circuit branches can become close to zero.
To increase the current, one could decrease the value of resistors Ra1 and Ra2. However, in this case, the threshold voltage variations of transistors Na1 and Pa1 strongly influence the current value in the circuit. Further, the decrease in the value of resistors Ra1 and Ra2 increases the power dissipation in these resistors, and thus the power consumption of the circuits.
Finally, due to deviation in MOS transistor threshold voltages, the output voltage may slightly vary between different circuits formed as in FIG. 1. Indeed, between extreme cases where transistors Na1/Na2 have a significant threshold voltage and where transistors Pa1/Pa2 have a low threshold voltage, and conversely, voltage OUT varies around value (Ra2×Vdd)/(Ra1+Ra2) since the voltage at node Aa varies.